Knowing?

"There are things we know we know, and that's helpful to know you know something. There are things we know we don't know and that's really important to know, and not think you know them when you don't. But the tricky ones are the unknown unknowns, the things we don't know we don't know. They're the ones that can get you in a bucket of trouble." Donald Rumsfeld (in an interview with David Frost, 27 June 2004)